The commercially available tools to evaluate images of normal tissues can display single functions overlaying a computer tomography (CT) image. Clinicians have demonstrated the clinical significance of knowing local regional tissue functions, i.e., more than one function. One simple example where the geometric overlay of more than one function is important is in the thorax. In the lung, if there is a region of good lung perfusion and poor ventilation, this is referred to as a shunt, where oxygenated blood exchanges gas locally in an environment rich in CO2 because the ventilation is poor.
The current state of the art tool for assessing the effects of radiation therapy on normal lung is a lung dose volume histogram. Marks et al. have suggested the mean lung dose is the critical value to examine, while Mary Graham et al. have suggested the V20 is the best predictor for toxicity. There are authors that have suggested even lower doses such as the V5 or V10 are important. R. Gopal et al. reported the V13 was a significant predictor of changes in the ability to exchange CO2 and O2 (DLCO). An older study by X. Liao et al. examined lung toxicity of partially irradiated mice. The mice were irradiated with slits that would only allow section of the mouse lung to be irradiated. The authors found when irradiating mice in this manner, the mid and lower lung irradiation produced significantly more toxicity than upper lung irradiation. This was one of the earliest works indicating a need for spatial information to predict toxicity.
The most recent volume of the International Journal of Radiation Oncology, Biology and Physics had a supplemental volume devoted to normal tissue toxicity. The guest editors R. Ten Haken and L. Marks introduce the term QUANTEC (quantitative analysis of normal tissue effects in the clinic). The vast majority of the supplement is a very thorough effort to refine the conventional approaches. One of the articles in this supplement refers to the need for functional imaging to quantify true changes in function and the dose levels when these changes occur.